The invention relates to compositions useful for producing polymer layers on printed wiring boards and other electronic and electrical interconnect devices. In many embodiments, the invention relates to photosensitive, positive-acting, aqueous-processable dry films made from those compositions. The films are particularly useful for forming a permanent dielectric on multilevel printed wiring boards.
Consumer demand for smaller, lighter electronic products has caused product manufacturers to require smaller semiconductor packages with increased levels of functional integration. These smaller, highly integrated packages require printed wiring boards having a large number of interconnection paths per unit board area. Frequently, multilayer printed wiring boards are required to achieve the xe2x80x9chigh densityxe2x80x9d of interconnection required to support these highly integrated packages.
Multilayer, high density printed wiring boards consist of alternating layers of electrically insulating polymeric material and metallic conductors which have been deposited on a copper-clad glass-epoxy substrate. Connections between metallic conductor layers are made through a plurality of tiny holes or xe2x80x9cviasxe2x80x9d in the polymer layer located between two conductor layers.
A two layer printed wiring board, for example, can be produced by first etching a circuit board pattern on the upper surface of a copper-clad glass epoxy substrate. An electrically insulating polymer layer is then deposited on the patterned surface, and holes are made through the polymer layer at desired points of conductor interconnection. An electrically conductive layer is then deposited on top of the polymeric layer, with the deposited conductive layer penetrating the vias to form electrical connections between the deposited layer and the circuit board traces on the glass epoxy substrate. A patterned conductive layer is then formed from the deposited conductive layer via a multistep photoetching process, resulting in two levels of circuit traces interconnected by the copper that was deposited within the vias.
Increased board interconnect density requires vias in a circuit board""s dielectric polymer layer that are smaller than can be economically achieved by conventional mechanical through-hole drilling. Vias smaller than what can be achieved by conventional mechanical drilling, typically 6 mils or less, are referred to as xe2x80x9cmicroviasxe2x80x9d. High density boards currently in production require microvias having diameters of about 3 to 5 mils.
Board manufacturers produce microvias by laser ablation, plasma etching, or photoimaging techniques.
Laser ablation is a sequential via formation technique. Vias are formed one at a time as a laser pulse is directed at a specific area of the circuit board. The etching process is anisotropic, which inherently limits resolution of the produced microvias. Because resolution is limited, and because sequential production processes are relatively slow compared to other processes, laser ablation is not preferred by board manufacturers.
Unlike laser ablation, photoimaging and plasma etching mass produce all of a given board level""s microvias simultaneously. Photoimaging techniques are more cost effective for mass producing high density wiring boards than plasma etching, and therefore are preferred by printed wiring board manufacturers for boards having high microvia densities.
In a photoimaging process, microvias are produced by positioning a pattern or xe2x80x9cmaskxe2x80x9d over a photosensitive polymer surface, exposing the masked polymer surface to actinic radiation, and then developing the surface to remove polymer from the board.
If the photosensitive polymer is made insoluble by exposure to actinic light, the coating is said to be xe2x80x9cnegative acting.xe2x80x9d In this case, the developer dissolves the unexposed polymer surface, leaving an image of polymer on the board that is a negative of the image on the mask. On the other hand, if the polymer is rendered soluble by exposure to actinic light, the polymer coating is said to be xe2x80x9cpositive acting.xe2x80x9d In a positive acting system, the developer removes the exposed polymer, leaving a positive image of the mask.
Positive acting polymeric materials are preferred in photopolymer applications because they provide improved resolution and yields.
Improved via resolution occurs because, unlike negative acting materials, positive acting materials do not swell during development.
If used in printed wiring board applications, board yields can be higher using positive acting systems for two reasons. First, if a dust particle is present during exposure in an unmasked area of a board bearing a negative acting polymer, the end result will be that an undesired via will be formed. As the undesired via will provide an undesired electrical connection between adjacent conductive layers of the board, the board is unusable. On the other hand, if the dust particle is present when a positive acting photosensitive polymer is exposed, the end result will be a spot of insulating dielectric material in an undesired location. Often, the presence of a small spot of additional dielectric will not spoil the board. Second, if an undesired spot will spoil a board manufactured using a positive acting material, the coating may be stripped from the board simply by developing under more severe conditions, and the layer can be remanufactured. Alternatively, the spot can be removed by reexposure to light followed by redevelopment of the board. Similar reworking is not practical with negative acting coatings because the coatings are rendered insoluble (and unworkable) by the exposure process. The use of positive acting materials can, therefore, greatly increase board yield. Consequently, board manufacturers prefer to work with a positive acting polymer system.
Another important factor in selecting a polymer system for printed wiring board manufacture is the type of material used to develop photoimaged boards. While many polymers will be soluble in a wide range of organic developers, the use of organic developers may present occupational hazards to workers. Furthermore, disposing of waste organic materials can be expensive. For these reasons it is preferred that polymer systems used in printed wiring board manufacture be developable in aqueous solutions.
Yet another requirement for polymer systems used to form a permanent dielectric layer on printed wiring boards is that the polymer system exhibit good mechanical properties at sufficiently high temperature to withstand subsequent board manufacture and soldering steps. One method to ensure good mechanical properties at high temperature is for the permanent dielectric layer to have a high glass transition temperature. Typically, the circuit boards will be required to withstand temperatures of at least 125 degrees Centigrade, and in some applications, as high as 200 degrees Centigrade. Polymer systems selected for board use must therefore have a relatively high glass transition temperature.
Polymer system selection also depends on the desired method of application of the polymer to the circuit board. Photosensitive polymer systems typically are applied to printed wiring boards as a liquid or as a dry film. In either case, an important requirement is that each cured polymer surface be essentially flat to enable the application of subsequent board layers. Such a flat surface is said to xe2x80x9cplanarxe2x80x9d or possess a high xe2x80x9cdegree of planarity.xe2x80x9d
Liquid polymer coatings are difficult to use in printed wiring board manufacture for at least two reasons. First, liquid coatings are difficult to apply and dry to the uniform thicknesses and degree of planarity desired by printed wiring board manufacturers. This problem is especially pronounced when manufacturing built up multilayer printed wiring boards where alternating layers of conductive and insulating materials are sequentially applied to the printed wiring board substrate. Additionally, liquid coatings can trap foreign objects such as dust particles and continue to trap such objects until the liquid has solidified.
A high degree of planarity is not easily obtained when applying a film over a board layer either, but improved planarity can be obtained by making the dry film layer relatively thick. The required thickness typically will be on the order of 1 mil or more. Films of this thickness or greater also are desirable because they provide good insulating properties and will provide a high degree of planarity.
Thick dry films of the type just discussed also are preferred because they can readily conform to circuit elements present on the circuit board. The dry film must conform to the circuit elements because any void formed between the applied polymer layer and the metallic circuit element will lead to board degradation when heat is applied to the board during subsequent thermal processing and soldering steps. Voids of this type also can lead to failure of the board or mounted devices after assembly. For these reasons many printed wiring board manufacturers prefer to manufacture printed wiring boards by pressure laminating dry film layers of polymeric coating precast on carrier sheets to printed wiring board substrates. Such laminating processes are not possible using liquid coatings.
Finally, dry film materials used to prepare printed wiring boards must exhibit favorable material handling characteristics when used in the typical printed wiring board preparation environment. For example, the dry film layer must be sufficiently pliable that it does not develop cracks when the dry film carrier sheet flexes during handling. The dry film layer also must maintain sufficient integrity to prevent flaking of material from the dry film layer, as such flaking can interfere with photoimaging of the dry film material.
Thus, printed wiring board manufacturers continue to search for improved polymer systems that can be used to form a permanent dielectric in printed wiring boards. The system should be capable of being applied to the circuit board as a dry film system, be positive acting, and be able to be developed in aqueous solutions. The system also should have a high Tg, be able to be applied in thicknesses greater than 1 mil so that it may readily conform to circuit board elements while providing a high degree of planarity, and be able to be photoimaged to produce microvias that can have a diameter of 3 mils or less. Preferably, the dry film material will be highly pliable and will not flake in use.
We have discovered that certain epoxy-based formulations can be used to produce positive acting, aqueous developable dry films and coatings useful for a wide range of applications related to the interconnection of electrical and electronic components.
The coatings and films of our invention are highly conformal to electrical circuit elements, exhibit high degrees of planarity in use, can be used in thickness in excess of 1 mil, and can provide good photoresolution. Preferred formulations are highly resistant to flaking and exhibit a high degree of pliability, rendering the coatings and films exceptionally easy to use in a printed wiring board production environment.
A first embodiment of our invention is a composition for forming a polymeric layer useful in the production of printed wiring boards. The composition includes a volatile organic carrier solution for mixing and casting the polymeric layer onto a surface, an epoxy resin in the amount of 20 to 60 weight percent of the nonvolatile components of the layer, a basic catalyst in the amount of 0.01 to 5 weight percent of the nonvolatile components of layer, and a film-forming polymer adduct totaling between 10 and 50 weight percent of the nonvolatile components of the layer. Preferably, the film-forming polymer adduct includes a polyfunctional nucleophile such as a diamine, a dianhydride or a novolac.
The composition can be made photosensitive by including a positive acting photosensitive agent, and can be used to prepare laminable dry films for printed wiring board production by casting the composition on a carrier sheet coated with a release agent, drying the cast composition to drive off most of the carrier solvent, laminating the carrier sheet bearing the dry film to a circuit board, removing the carrier sheet, patterning the deposited dry film material that remains adhered to the circuit board, and heating the patterned composition to cure the thermsetting composition. Reactive liquid polymers, stress-modified phenolic resins, or rubbers can be added to the composition to increase pliability and reduce flaking of the cast and dried dry film material.
In another embodiment of our invention, a laminable dry film able to be developed in aqueous alkaline solution and useful for forming a permanent dielectric area on a printed wiring board surface includes a first thermosetting polymeric layer for adhering the film to the printed wiring board surface and for encapsulating circuit board elements. A second thermosetting polymeric layer is adhered to the first layer and includes a positive-acting photosensitive agent for increasing the solubility of the second layer in aqueous base after exposure to actinic radiation. The thermosetting film layers are formed from the compositions just described, and the bilayer films are capable of producing printed wiring board dielectric areas having a high degree of planarity and via resolution of 3 mils or less.
In still another embodiment of the invention, a structure is formed by laminating a film such as the film described above to a printed wiring board already having a patterned conductive layer on its surface. A multilayer printed wiring board then can be formed by exposing the laminated film to actinic light through a photomask, developing the exposed film to remove the first and second polymeric layers from the exposed board area to form microvias, depositing an electrically conductive layer onto the developed surface of the board, and patterning the conductive layer deposited on the board to form a second patterned conductive board layer interconnected to the first.
In yet another embodiment of the invention, a process for making a bilayer film is disclosed in which a first film layer is cast on to a first carrier sheet, a second polymer layer is cast on a second carrier sheet, and then the two film layers joined together. This process can result in better drying of film layers and enable the use of certain film layer compositions that could not be used if the film layers were sequentially applied to a carrier sheet and then dried.
Another embodiment of the invention is a process for plating a printed wiring board made from a photosensitive dry film material. In this process, an electrically conductive layer is deposited on a partially cured, roughened surface. By roughening a partially cured surface prior to plating, it is possible to obtain better metal to polymer adhesion.
Other embodiments of the invention incorporate sacrificial or non-etchable fillers in the dry film material to promote adhesion of metal to photoprocessed boards made from our films and coatings.
The following detailed descriptions of our invention illustrate how the invention can be used in a dry film useful for forming permanent dielectric areas on built-up multilayer circuit boards. While the description will focus on the use of the invention in the form of photosensitive film layers that can be used alone or in conjunction with similar nonphotosensitive film layers to make built-up multilayer circuit boards, those of ordinary skill will realize that the invention will be useful in a wide variety of coating or film applications, such as a photoresist, soldermask, flexible circuit coverlay, the production of substrates for devices such as ball grid arrays, or in any other application where a photoimageable polymer is required to be applied to a substrate.
In its broadest embodiment, our invention comprises thermosetting compositions that can be cast onto or otherwise applied to a carrier sheet or other substrate to form an uncured thermosetting polymeric layer of material. Addition of a photosensitive agent to the composition will yield a photoprocessable film layer. The film may have any thickness from about 0.1 to 10 mils, although a total film thickness of at least 1.5 mils is currently preferred by the printed wiring board industry to provide adequate dielectric properties and to provide for a planar upper film surface. Maximum film thickness typically will be limited by the resolution required in the particular application. In built-up multilayer board applications, the film thickness usually does not need to exceed about 4 mils.
While the invention may take the form of a single layer of photosensitive thermosetting material, we have found that it is preferred to produce photoprocessible films in accordance with the invention as bilayered, thermosetting films. In these embodiments, the invention takes the form of a release-coated polyester carrier film onto which are applied a first, photosensitive (PS) layer, and a second nonphotosensitive (NPS) layer.
Solutions useful for casting both photosensitive and non-photosensitive layers of one such bi-layer film contain at least a carrier solvent, a thermosetting epoxy component which typically will include a polyfunctional epoxy resin, a comonomer such as phthalic anhydride, and a basic catalyst or other curing agent. Optional components include a film-forming thermoplastic, a film-forming polymer adduct such as the product of the reaction between a polyisocyanate and a base-soluble phenolic or cresylic novolac having a large stoichiometric excess of the novolac, one or more comonomers for the epoxy resin, an aminoplast such as melamine, and, in a photosensitive layer, a photosensitive compound that increases the solubility of the film layer in aqueous base after exposure to actinic radiation.
Although not required by the invention, solutions used for casting films may also include a wide variety of additives such as organic and inorganic fillers, adhesion promoters, thermal stabilizers, colorants, viscosity control agents, wetting agents, and flexibilizers.
Carrier solvents useful for preparing polymer mixtures for casting film layers in accordance with the invention include virtually any organic solvent or mixtures thereof that are capable of dissolving or solubilizing film layer starting materials and which can be removed substantially by drying the film layer at relatively low oven temperature settings of between about 25 and 150 degrees Centigrade. Preferred oven temperature settings as measured by the air temperature in the oven are between 75 and 140 degrees Centigrade. It should be noted that to produce optimal dried films, a combination of proper solvent choice, oven temperature settings, air flow settings and residence time is required. It should also be noted that the air flow in the oven will significantly impact film drying and may allow properly dried films to be produced even though the boiling point of the solvent may exceed the oven temperature setting. Operating conditions for any particular solvent used in a film composition should be empirically determined by varying oven temperature, air flow and residence time and comparing the results.
Preferred carrier solvents include acetone or other acyclic ketones including methyl ethyl ketone and the like, hydrocarbons such as hexane, cyclohexane, heptane, toluene and the like, esters such as ethyl acetate, isobutyl acetate, methyl proprionate, ethyl ethoxy propionate, propylene glycol ethyl ether acetate, ethyl lactate and the like, and polar aprotic solvents such as gammabutyrolactone, N-methylpyrrolidone, and similar compounds. Other information concerning the selection and use of solvents which may be useful in photosensitive film systems is contained in U.S. Pat. No. 5,128,230, the disclosure of which is hereby incorporated by reference.
The process of casting and drying a film on a carrier sheet normally leaves some carrier solvent remaining in the film. Preferably the retained carrier solvent amounts in the dried film are from about 0.1 to 5 weight percent retained solvent. Depending on what effects are desired, a properly dried film may have retained solvent toward either the higher or lower end of this range. If stable film processing immediately after film casting is desired, for example to provide more stable image development properties, then it is generally preferred to have less retained solvent. If improved film flexibility or laminability is desired, then it is generally preferred to have more retained solvent. We have found that films having the composition of Example 5, below, when cast and dried in an oven using different residence times, produced films having different amounts of retained solvent and strain characteristics. Films having up to 2.2% retained solvent exhibited less than 0.4% strain before failure, as measured by the mandrel test described later in this application, while a film containing 3.5% retained solvent withstood 0.5% strain before failure. Higher retained solvent also may cause increased erosion in freshly cast films, but after aging, film erosion becomes independent of retained solvent amount.
In a relatively thick film containing temperature-sensitive moieties such as the photosensitive moiety, it may be difficult to find drying conditions which give both low residual solvent and do not initiate decomposition of the photosensitive moiety. Thus, it is apparent to one skilled in the art that retained solvent is not the sole measure of a properly dried film. Nevertheless, it may be said that more preferred retained solvent amounts in the dried film are from about 0.3 to 4 weight percent retained solvent. Most preferred retained solvent amounts are from 0.5 to 3.0 weight percent retained solvent. As used in this application, a xe2x80x9cdriedxe2x80x9d film is a film that contains less than 5 weight percent retained solvent after drying as measured by total film weight.
Typical film drying conditions for webs having widths on the order of 1 to 6 feet or more will include web speeds of 5 to 500 feet per minute, drying temperatures of 65 to 150 degrees Centigrade, and air flows on the order of a few thousand feet per minute. Operating conditions preferably should be adjusted to yield a non-tacky film unless an interleaf is used when rolling up the dried film to keep tacky film surfaces separated.
Film-forming polymer adducts useful in the invention will be those that will cause the cast solution to produce a relatively uniform coating on a substrate or traveling web which will maintain its integrity while being dried or devolitalized. As used in this application, the term xe2x80x9cfilm-formingxe2x80x9d describes the ability of a solution of plastic materials to uniformly wet a traveling web or substrate and to maintain integrity of the dried film during manufacture and use. In preferred embodiments of the invention, the adduct is formed prior to casting of the film material. Preferred film-forming adducts are urethanes. In forming a film-forming urethane adduct such as from a polyisocyanate and a phenolic novolac, it is preferred to use a stoichiometric excess of phenolic novolac. The mole equivalent excess of novolac should be an amount effective to prevent gelation of the adduct prior to casting the film. Typically, the mole equivalent ratio of phenolic to isocyanate moieties will be at least 5, and preferably is between 10 and 50. If a reactive liquid polymer is employed as discussed in detail below, the mole equivalent of the reactive liquid polymer should be added to the mole equivalent of isocyanate when calculating the novolac to isocyanate ratio. The film-forming adduct constituents should comprise in total between about 10 and 50 weight percent of a film layer""s non-volatile components, and preferably will comprise between about 20 and 40 weight percent on that basis.
Film-forming thermoplastics useful in the invention will be those that that can be mixed with the other film components in the carrier solvent and dried without the mixture becoming heterogeneous. The film-forming thermoplastic or mixtures thereof should comprise in total between about 0 and 25 weight percent of a film layer""s non-volatile components, and preferably will comprise no more than 5 weight percent on that basis.
Virtually any catalyst known to catalyze copolymerization of the epoxy moietes with nucleophilic curing agents, without interfering with the polymerization of other film constituents, may be used in the invention. Preferred catalysts are heterocyclic amines, tertiary aromatic amines and tertiary-amino phenols with tertiary-amino phenols being more preferred. In many embodiments, the use of 2, 4, 6-tris-[(dimethylamino)methyl] phenol is most preferred. Mixtures of catalysts may be used and in many cases may be preferred. It should be noted that film constituents such as diamines, which are included in the film for primary functions other than their catalytic activity, may perform as catalysts in the film-forming mixture. Catalyst typically should comprise in total up to about 10.0 weight percent of a film layer""s non-volatile components, and preferably will comprise between about 0.01 and 5.0 weight percent on that basis.
Epoxy resins useful in the invention include most epoxy resins having two or more glycidyl ether groups. The epoxy should be soluble in the carrier solvent and be capable of polymerization with the other epoxy resins or comonomers used in the film under the desired printed wiring board curing conditions. Exemplary resins include aromatic glycidyl ethers, aliphatic glycidyl ethers, and other resins having at least diglycidyl ether functionality. Preferred epoxy resins are aromatic and aliphatic glycidyl resins having two or more functional groups per molecule. The epoxy resin or mixtures thereof should comprise in total between about 20 and 60 weight percent of a film layer""s non-volatile components, and preferably will comprise between about 40 and 60 weight percent on that basis.
Comonomers for epoxy resins also may be included in the film layers of the invention. These comonomers should be selected for compatibility with the other film layer constituents, and can be any of a wide number of comonomers known in the art. The comonomers should have a functionality of two or more under drying and curing conditions. Comonomers containing the following moieties are useful in the invention: Lewis acids and bases, acid anhydrides, primary and secondary amines, acids, esters, alcohols, phenols, epoxides, mercaptans, isocyanates, melamine-, urea-, and phenol-formaldehyde resins. Comonomers such as carboxylic acid anhydrides and dianiline derivatives are preferred. In many embodiments, the use of a comonomer such as phthalic anhydride is most preferred. It may be useful to use two comonomers in combination. Comonomers or mixtures thereof should comprise in total between about 0 and 25 weight percent of a film layer""s non-volatile components, and preferably will comprise between about 1 and 10 weight percent on that basis.
The short-term thermal resistance of film layers, as measured by glass transition temperature, is a key property of built-up multilayer dielectric layers in high density circuit boards. The fabricated high density circuit board must have sufficient thermal resistance to survive circuit assembly and use conditions unchanged. For a built-up multilayer circuit, this implies a Tg at least equal to the Tg of the substrate. This is only about 130xc2x0 C. for an FR-4 substrate. For chip-on-board applications, the interconnect is made by wirebonding the chip to the substrate. This in turn requires a Tg greater than the wirebonding temperature of about 170 to 190xc2x0 C. In other applications such as flip chips, a sufficiently high Tg may be required to survive the solder reflow temperatures used to mount the flip chip.
The formulation of epoxy resins to meet end use requirements is a complex art. However, those skilled in the art recognize that there are certain guiding principles. Three points to consider are the rigidity of the resin backbone, the perfection of the network, and that the Tg of the epoxy network is a function of the crosslink density. Perfection means a stoichiometric balance of epoxy resin to comonomers and hardeners. We define the stoichiometric ratio to be:
ratio=[(mole equivalents of epoxy)xe2x88x92(mole equivalents of comonomer)]/[(mole equivalents of novolac)xe2x88x92(mole equivalents of isocyanate)].
where the comonomers are, for example, phthalic anhydride, diaminodiphenyl sulfone, and the like. This ratio hereafter will be referred as the xe2x80x9cepoxy to novolac ratio.xe2x80x9d Each film layer optionally includes one or more amino resins or xe2x80x9caminoplastsxe2x80x9d . Examples of aminoplasts useful in the invention include urea-formaldehyde adducts, melamine-formaldehyde adducts and glycoluril derivatives, with the glycolurils and melamine-formaldehyde adducts being especially preferred. Preferably, the amino resin should be capable of reacting with film-forming adduct constituents such as the novolac or glycidyl ethers during low temperature drying of the manufactured film. The amino resin or mixtures thereof should comprise in total up to about and 10 weight percent of a film layer""s non-volatile components, and preferably will comprise between about 0.1 to 2 weight percent on that basis. It is believed that the use of an aminoplast improves image stability during the initial stages of cure.
Photosensitizing compounds useful in the invention include any positive-acting photosensitizers containing an ortho-quinone diazide group. Such compounds include 1,2-naphthoquinone-2-diazido-5-sulfonyl chloride and its esters with phenol, phenolic derivatives, phenolic polymers and the like. Especially preferred are esters of di-and tri-hydroxybenzophenone. Photosensitizing compound should comprise in total between about 2 and 20 weight percent of a photosensitive film layer""s non-volatile components, and preferably will comprise between about 5 and 15 weight percent on that basis.
It is generally required that the foregoing compositions be soluble in aqueous base, as the development of photoactivated film layers in accordance with the invention is conducted in aqueous basic developer, typically at a pH of about 11 to 15, and more preferably in a range of 12.5 to 13.5.
We believe that the photoactive compound functions by increasing the solubility of the film in the exposed areas and decreasing the solubility of the film in the unexposed areas. The differential solubility between exposed and unexposed areas is the basic requirement for forming a relief image by exposure and development. More specifically, the presence of photoactive compound in the film creates differential solubility in the film after exposure by increasing the dissolution rate of the film in the exposed areas and decreasing the dissolution rate in the unexposed areas. However, the dissolution rate in the unexposed area is not zero and some film thickness will be lost during development. The amount of film thickness lost during via development is referred to as the film erosion.
It is desirable that the loss of film thickness (erosion) during image development be minimized. The erosion in a film having a single photosensitive layer (PS) is the film thickness lost from the unexposed region during the develop time required to open vias in the exposed region.
Erosion in a bilayer film is more complex because the imaged region consists of both the exposed photosensitive layer and the underlying non-photosensitive (NPS) layer. The NPS layer does not have a solubility differentiation between the exposed and unexposed regions. The erosion in a film having a bilayer construction is the film thickness lost from the unexposed region during the development time required to form a via by dissolving material from the exposed region of the PS layer and the underlying NPS layer. To have satisfactory low erosion in a bilayer film requires an NPS layer having significantly higher solubility than the unexposed PS layer. If the NPS layer does not have significantly higher solubility than the unexposed PS layer, the dissolution rates of the exposed and unexposed areas will become comparable when the developing feature reaches the NPS layer and significant erosion will result. A preferred bilayer film will have a PS layer that develops with low erosion at conditions suitable for rapidly developing the NPS layer. NPS layers can be formulated in the same manner as PS layers, except, of course, for the absence of the photosensitive agent.
While not wishing to be bound by any particular theory, it is believed that the components of our film and coating materials form a polymer network during their manufacture and cure. During drying, it is believed that the aminoplast reacts with itself and with novolac species, thereby stabilizing the dry film. As temperature increases in drying and curing steps, various epoxy-amine, epoxy-phenolic and epoxy-comonomer reactions occur, resulting in the ultimate formation of a polymer network layer useful as a permanent dielectric.